1. Field of the Invention
The invention relates to a control system for a direct injection-spark ignition type of engine equipped with an exhaust gas recirculation system, and, in particular, to a direct injection-spark ignition engine control system which has an exhaust system with a lean NOx catalyst for controlling an emission level of nitrogen oxides in exhaust gas produced as a result of combustion of a fuel mixture of .lambda.&gt;1 and provides a stable nitrogen oxide reduction efficiency of the lean NOx conversion catalyst.
2. Description of the Related Art
Engine control system of this type incorporate in an exhaust line an NOx adsorption type of lean NOx conversion catalyst which, on one hand, adsorbs NOx in the exhaust gas while the air-fuel mixture is leaner than a stoichiometric mixture and, on the other hand, desorbs or releases the NOx into exhaust gas for catalyzing reduction of the NOx while the air-fuel mixture is richer than a stoichiometric mixture. As is known from, for example, International Patent Application WO93/07363, such an engine control system controls the engine to operate with an enriched mixture under accelerating conditions or full loading operating conditions and with a lean mixture under the remaining operating conditions, so as to improve specific fuel consumption.
An engine control system for a direct injection-spark ignition type of engine known from, for example, Japanese Unexamined Patent Publication 7-119507 controls the engine to cause stratified charge combustion in a lower engine loading zone and homogeneous charge combustion in a high engine loading zone. While the engine operates with lower speeds in the high loading zone, a given amount of fuel is delivered in two steps through early and late split injection in a intake stroke, so as to diffuse a first half of fuel sprayed through the early split injection in the combustion chamber before the end of a intake stroke and the second half of fuel in the combustion chamber with its volume increased, This prevents a generation of rich or dense mixture over the top of a piston in a subsequent compression stroke that generally occurs when a given amount of fuel is sprayed all at once through non-split injection, which is desirable to prevent generation of smoke.
Another engine control system for a direct injection-spark ignition engine cooperates with a fuel injector which is direct to face the top of a piston and energized to spray a small amount of fuel preparatorily at the beginning of a intake stroke when the engine causes knocking. The fuel partly sticks to the top wall of the piston on a side of an intake port and bounces off the piston wall toward the intake port to cool the piston head and the combustion chamber on the intake port side with the heat of vaporization of the fuel. Such an engine control system is known from, for example, Japanese Unexamined Patent Publication 7-217478.
An NOx adsorption type of lean NOx conversion catalyst described in the Japanese Unexamined Patent Publication 7-119507 causes aggravation of its catalytic conversion efficiency due to an increase in the amount of NOx adsorption when the engine continues lean charge combustion. The engine control system controls the engine to make enriched charge combustion to force the lean NOx conversion catalyst to desorb NOx and catalyzes reduction of NOx, so as thereby to refresh it with an effect of keeping stabilized catalytic conversion efficiency. Generally, emission levels of reducing hydrocarbons (HC) and reducing carbon monoxide (CO) into the exhaust gas are increased as a fuel mixture is enriched even more, desorption of NOx from the lean NOx conversion catalyst and reduction of the NOx progress within a short period of time. For that reason, the prior art engine control system controls the air-fuel ratio to significantly lower to approximately 12 to 13 so as to enrich a fuel mixture. Because, although the engine is operative with a lean mixture, an enriched fuel mixture is provided only for the purpose of refreshing the lean NOx conversion catalyst, the fuel efficiency is aggravated. Further, a fuel mixture is greatly enriched regardless of driver's intention, this is always accompanied by a great change in engine output which is unpleasant for the driver.